The present invention relates to a developing device with respect to electrostatic latent images, in which non-magnetic toners of mono-component are provided on the electrostatic latent images to be made visible. The developing device of the invention may be applied to an electrographic copying machine, or a recording machine for electrostatic latent images.
There is known a mono-component developing method in which a thin layer of charged toners uniformly formed on a toner transport member is brought into contact with a photosensitive member or a photoreceptor to develop electrostatic latent images (e.g. Japanese Patent Laid-Open No. 143831/1977).
There are many systems for a mono-component developing method. In principle, a cylindrical toner-transport member is set between a photoreceptor drum on which electrostatic latent images are formed and a mono-component toner container. A toner-levelling member, which plays a role in charging toners, is pressed against the toner transport member. Toners are charged positively or negatively to an adequate level while passing through between the toner transport member and the toner levelling member. At the same time, a thin layer of charged toners is formed on the toner transport member and the toners are transported to the photoreceptor and attracted electrostatically to electrostatic latent images on the photoreceptor to be made visible.
However, a mono-component developing system can not form solid copied images of high density while keeping a specified image density of line images.
The transfer of toners on a toner transport member to a photosensitive member (photoreceptor) is forced by the difference of electric potential represented by an absolute value of (Vo-Vb) (hereinafter referred to as ".DELTA.V" between a bias potential applied to the toner transport member (Vb) and the surface potential of the receptor (Vo)).
When a high electrically resistant sleeve (hereinafter, referred to as "high resistant sleeve") is applied to a mono-component developing system as a toner transport member, fine lines can be reproduced in high density in spite of small .DELTA.V, taking advantage of edge effects. On the other hand, when solid images are reproduced in high density, large .DELTA.V is needed. Therefore, when fine lines are reproduced at such large .DELTA.V as solid images are reproduced in high density, there are formed smoothless and flatless line images constituted of an excessive amount of toner. The application of a high resistant sleeve can not achieve the compatibility of the reproductions of both fine line images with sharp edges and solid images of high density.
When a low electrically resistant sleeve (hereinafter, referred to as "low resistant sleeve") is applied to a mono-component developing system as a toner transport member, solid images can be reproduced in high density at narrower .DELTA.V than a high resistant sleeve. However, because a low resistant sleeve display little edge effects, it needs large .DELTA.V for the reproduction of fine lines with sharp edges. Further, higher bias voltage is applied than necessary in order to get large .DELTA.V, a photoreceptor and a toner transport member and the like are affected adversely.
A mono-component copying machine is generally used at low copying speed. Even if the copying speed is simply adjusted high, copied images with sufficient density are not formed.